Compression testers are used to measure the compression pressure in an individual cylinder of an internal combustion engine. A typical compression tester consists of a compression pressure gauge with a pressure measurement range of, for example, 0-300 pounds per square inch (psig), and a compression pressure hose assembly that attaches to the compression pressure gauge. Typically, the compression tester hose assembly connects to a spark plug socket in an engine cylinder head. Thus, the spark plug is removed from the cylinder head, and the compression tester hose assembly is threaded into the spark plug hole, or socket, in order to establish fluid communication between the compression pressure gauge and the engine cylinder. The engine is then cranked for several revolutions, typically using the engine starter, in order to measure the maximum compression pressure in the engine cylinder.
The compression pressure is used to test internal combustion engines for proper compression in each cylinder. Improper compression can cause rough idling or running, backfiring and poor fuel economy. Low compression in a cylinder can be an indication of worn or damaged piston rings, valves, head gasket or timing chain. A high compression reading may indicate carbon buildup in the cylinder. If the compression pressure gauge remains at the same value for several strokes and then begins to climb, the cylinder may have a sticking valve.
Standard spark plug sockets typically include 14-millimeter (mm) nominal threads, 18-mm nominal threads, 12-mm nominal threads or 10-mm nominal threads. However, some newer engines utilize other spark plug thread sizes. For example, the Ford Triton™ three-valve V8 engine family utilizes 12-mm nominal threads.